For example, Patent Literature 1 discloses a failure detection device for an air-fuel ratio control device, which has air-fuel ratio sensors disposed respectively in front and behind a catalyst. The device determines a failure of the air-fuel ratio sensor installed upstream or a failure of a catalyst convertor, based on an output difference between the air-fuel ratio sensors in front and behind the catalyst. Further, in the device, an output from the air-fuel ratio sensor at a downstream side is corrected based on a reference output, and an output from the air-fuel ratio sensor at an upstream side is corrected by using the air-fuel ratio sensor at the downstream side.